In mobile communication, ARQ (Automatic Repeat Request) is applied to downlink data from a radio communication base station apparatus (hereinafter abbreviated to “base station”) to radio communication mobile station apparatuses (hereinafter abbreviated to “mobile stations”). That is, mobile stations feed back response signals representing error detection results of downlink data, to the base station. Mobile stations perform a CRC (Cyclic Redundancy Check) of downlink data, and, if CRC=OK (no error), feed back an ACK (ACKnowledgement), and, if CRC=NG (error present), feed back a NACK (Negative ACKnowledgement), as a response signal to the base station. These response signals are transmitted to the base station using uplink control channels such as a PUCCH (Physical Uplink Control CHannel).
Also, the base station transmits control information for reporting resource allocation results of downlink data, to the mobile stations. This control information is transmitted to the mobile stations using downlink control channels such as L1/L2 CCH's (L1/L2 Control CHannels). Each L1/L2 CCH occupies one or a plurality of CCE's. If one L1/L2 CCH occupies a plurality of CCE's (Control Channel Elements), the plurality of CCE's occupied by the L1/L2 CCH are consecutive. Based on the number of CCE's required to carry control information, the base station allocates an arbitrary L1/L2 CCH among the plurality of L1/L2 CCH's to each mobile station, maps control information on the physical resources associated with the CCE's (Control Channel Elements) occupied by the L1/L2 CCH, and performs transmission.
Also, to use downlink communication resources efficiently, studies are underway to associate PUCCH's with CCE's on a one-to-one basis. According to this association, each mobile station can decide the PUCCH to use to transmit a response signal from that mobile station, from the CCE corresponding to physical resources on which control information for that mobile station is mapped. That is, each mobile station maps a response signal from the subject mobile station on a physical resource, based on the CCE corresponding to physical resources on which control information for that mobile station is mapped.
Also, as shown in FIG. 1, studies are underway to perform code-multiplexing by spreading a plurality of response signals from a plurality of mobile stations using ZC (Zadoff-Chu) sequences and Walsh sequences (see Non-Patent Document 1). In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence having a sequence length of 4. As shown in FIG. 1, in a mobile station, first, an ACK or NACK response signal is subject to first spreading to one symbol by a ZC sequence (having a sequence length of 12) in the frequency domain. Next, the response signal subjected to first spreading is subject to an IFFT (Inverse Fast Fourier Transform) in association with W0 to W3. The response signal spread in the frequency domain by a ZC sequence having a sequence length of 12 is transformed to a ZC sequence having a sequence length of 12 by this IFFT in the time domain. Then, the signal subjected to the IFFT is subject to second spreading using a Walsh sequence (having a sequence length of 4). That is, one response signal is allocated to each of four symbols S0 to S3. Similarly, response signals of other mobile stations are spread using ZC sequences and Walsh sequences. Here, different mobile stations use ZC sequences of different cyclic shift values in the time domain or different Walsh sequence. Here, the sequence length of ZC sequences in the time domain is 12, so that it is possible to use twelve ZC sequences of the cyclic shift values “0” to “11,” generated from the same ZC sequence. Also, the sequence length of Walsh sequences is 4, so that it is possible to use four different Walsh sequences. Therefore, in an ideal communication environment, it is possible to code-multiplex maximum forty-eight (12×4) response signals from mobile stations.
Here, there is no cross-correlation between ZC sequences of different cyclic shift values generated from the same ZC sequence. Therefore, in an ideal communication environment, a plurality of response signals subjected to spreading and code-multiplexing by ZC sequences of different cyclic shift values (0 to 11), can be separated in the time domain without inter-code interference, by correlation processing in the base station.
However, due to the influence of, for example, transmission timing difference in mobile stations, multipath delayed waves and frequency offsets, a plurality of response signals from a plurality of mobile stations do not always arrive at a base station at the same time. For example, if the transmission timing of a response signal spread by a ZC sequence of the cyclic shift value “0” is delayed from the correct transmission timing, the correlation peak of the ZC sequence of the cyclic shift value “0” may appear in the detection window for the ZC sequence of the cyclic shift value “1.” Further, if a response signal spread by the ZC sequence of the cyclic shift value “0” has a delay wave, interference leakage due to the delayed wave may appear in the detection window for the ZC sequence of the cyclic shift value “1.” That is, in these cases, the ZC sequence of the cyclic shift value “1” is interfered by the ZC sequence of the cyclic shift value “0.” Therefore, in these cases, the separation performance degrades in a response signal spread by the ZC sequence of the cyclic shift value “0” and a response signal spread by the ZC sequence of the cyclic shift value “1.” That is, if ZC sequences of adjacent cyclic shift values are used, the separation performance of response signals may degrade.
Therefore, up till now, if a plurality of response signals are code-multiplexed by spreading using ZC sequences, a difference of cyclic shift values (i.e. a cyclic shift interval) is provided between the ZC sequences, to an extent that does not cause inter-code interference between the ZC sequences. For example, when the difference between the cyclic shift values of ZC sequences is 2, only six ZC sequences of the cyclic shift values “0,” “2” “4,” “6,” “8” and “10” amongst twelve ZC sequences of the cyclic shift values “0” to “11,” are used in first spreading of response signals. Therefore, if Walsh sequences having a sequence length of 4 are used in second spreading of response signals, it is possible to code-multiplex maximum 24 (6×4) response signals from mobile stations.    Non-Patent Document 1: Multiplexing capability of CQIs and ACK/NACKs form different UEs (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-072315.zip)